An automatic transmission is configured to automatically switch shift gear stages, that is, power transmission paths by selective engagement of a plurality of hydraulic friction engaging elements, each of the power transmission paths being constituted by a planetary gear mechanism and the like. Each of the shift gear stages is basically formed by the engagement of two friction engaging elements. However, a first gear stage in a D range (drive range) is conventionally formed by one friction engaging element and an OWC (one-way clutch) for the purpose of, for example, smoothing of a gear shift operation.
However, the OWC is high in cost. In addition, the OWC incurs rotational resistance at gear stages other than the first gear stage in the D range, and this hinders improvement of fuel efficiency of an engine. On this account, discontinuation of the OWC has been proposed or executed in recent years.
In this case, the first gear stage is formed by, for example, engaging a friction engaging element such as a low clutch engaged at predetermined low shift gear stages including the first gear stage and a friction engaging element such as a low reverse brake engaged at the first gear stage and a reverse gear stage. The shifting to the first gear stage is performed by engaging the latter friction engaging element in a state where the former friction engaging element is engaged. Therefore, to successfully shift to the first gear stage from the other shift gear stage, it is necessary to precisely perform a control operation of a timing of the engagement of the friction engaging element such as the low reverse brake and a control operation of engaging power of the friction engaging element.
To solve such problems, PTL 1 discloses a low reverse brake using a tandem hydraulic actuator including two pistons.
As shown in FIGS. 11A and 11B, a low reverse brake A is configured such that a plurality of friction plates D are arranged between a transmission case B and a rotary member C accommodated in the case B, the friction plates D being alternately splined to an inner peripheral surface of the case and an outer peripheral surface of the rotary member. Further, the low reverse brake A includes: a pressing piston F configured to press the friction plates D against biasing force of a return spring E; and a clearance adjusting piston G provided behind the pressing piston F for adjustment of a clutch clearance.
According to the low reverse brake A, when the low reverse brake A is not engaged, as shown in FIG. 11A, each of the pressing piston F and the clearance adjusting piston G is held at a retreated position by the biasing force of the return spring E, and a relatively large clutch clearance is generated between a tip end of the pressing piston F and a stopper member H, the clutch clearance being obtained by subtracting the sum of thicknesses of the friction plates D from a distance between the tip end of the pressing piston F and the stopper member H. In this state, by applying oil pressure to the clearance adjusting piston G, as shown in FIG. 11B, the piston G and the pressing piston F moves forward against the biasing force of the return spring E to a stroke end of the clearance adjusting piston G. With this, the clutch clearance becomes smaller than that shown in FIG. 11A by a distance of the forward movement of each of the pistons F and G.
The oil pressure is previously supplied to an oil pressure chamber (hereinafter referred to as a “clearance adjusting chamber”) I of the clearance adjusting piston G. With this, when the oil pressure is supplied to an oil pressure chamber (hereinafter referred to as a “pressing chamber”) J of the pressing piston F for engaging the low reverse brake A, the low reverse brake A is engaged with good responsiveness. Thus, the timing of the engagement and the engaging power can be precisely controlled.